The present invention relates to a step-down chopper-type switching power supply unit of a non-insulated type, in which the input side and the output side are not electrically insulated from each other. More particularly, the present invention relates to a switching power supply unit with which the power consumption can be reduced in a light-load operation and in a stand-by operation, and also to a semiconductor device for such a switching power supply.
A conventional switching power supply unit will be described with reference to the drawings. FIG. 6 illustrates a schematic circuit configuration of a non-insulated step-down switching power supply unit, and FIG. 7 illustrates current/voltage waveforms during the operation of the switching power supply unit.
The general configuration and the operation of the switching power supply unit illustrated in FIG. 6 will now be described.
First, before the start-up of a control circuit 100, a switch 101 in the control circuit 100 is in a closed state, whereby an internal circuit current supply circuit 102 and a control circuit power supply capacitor 103 are connected to each other.
When an input voltage VIN is applied to a main input terminal 104, a control current flows from the internal circuit current supply circuit 102 via the switch 101 to the control circuit power supply capacitor 103, thereby increasing a power supply voltage Vcc of the control circuit 100. When the power supply voltage Vcc reaches the start-up voltage of the control circuit 100, a start-up/shut-down circuit 105 starts operating and the control circuit 100 starts up.
When the start-up/shut-down circuit 105 starts operating, the switch 101 is opened, thereby stopping the current supply from the internal circuit current supply circuit 102 to the control circuit power supply capacitor 103. Thereafter, a control current is supplied to the control circuit 100 from the control circuit power supply capacitor 103. At this point, an output voltage VOUT at a main output terminal 106 is 0 V.
Then, the control circuit 100 starts operating, thereby starting the switching (i.e., ON/OFF) operation of a switching device 107, which is an N-channel MOSFET. When the switching device 107 is ON, a drain current IDS flows from the main input terminal 104 via the switching device 107 into a coil 110a of a voltage conversion circuit 110. Then, when the switching device 107 is turned OFF, the electric energy accumulated in the coil 110a is supplied to the main output terminal 106 via a regenerative diode 110b, thereby increasing the output voltage VOUT at the main output terminal 106.
When the switching device 107 is turned OFF, with the output voltage VOUT having increased to be greater than the sum of a breakdown voltage Vz of a Zener diode 111, which is an output voltage detection circuit, a forward voltage Vf of a diode 112, which is a feedback circuit, and a power supply voltage Vcc of the control circuit 100, i.e., Vz+Vf+Vcc, a current flows into the control circuit power supply capacitor 103 from the main output terminal 106 via the Zener diode 111 and the diode 112. As a result, the voltage value of the output voltage VOUT is fed back to the control circuit 100, and the power supply voltage Vcc is supplied from the main output terminal 106 to the control circuit 100.
When the power supply voltage Vcc is increased by the voltage supplied from the main output terminal 106 to the control circuit 100, and the power supply voltage Vcc reaches a predetermined value, a switching element 114 is turned ON by a shunt regulator 113, and a PWM control current IPWM is supplied from the control circuit power supply capacitor 103 to a resistor 115, whereby a voltage is applied across the resistor 115.
The on-duty ratio of the switching device 107 is determined by the voltage across the resistor 115 and the output signal of a comparator 117, which receives a triangular-wave signal from an oscillator 116, whereby the pulse width to be applied to the switching device 107 is determined.
As described above, the conventional switching power supply unit variably controls the duty ratio of the switching device 107, thereby controlling the output voltage VOUT at the main output terminal 106 to be a predetermined value.
As described above, the conventional switching power supply unit employs a pulse width modulation (PWM) method for improving the precision of the output voltage VOUT. Typically, a switching frequency fc in the PWM method is set to a constant value of about 100 kHz to 200 kHz, for example. Moreover, the comparator 117 determines the on-duty ratio xcex4 of the switching device 107 as described above, and operates with a constant frequency and with the minimum on-duty ratio during a light-load operation, including a stand-by operation, as illustrated in FIG. 7.
In recent years, there is a demand for reducing the amount of energy use, i.e., energy conservation, from an environmental point of view. Thus, there is a demand for further reduction in the power consumption and further improvement in the efficiency for power supply units, particularly for switching power supply units.
However, in the conventional switching power supply unit, the switching device 107 is in the switching operation regardless of the magnitude of the load. Therefore, power is constantly wasted while being supplied to, and consumed by, the main output terminal 106. Moreover, there is an increased switching loss because the switching frequency fc is relatively high, i.e., 100 kHz or more.
The present invention has been made in order to solve these problems in the prior art, and has an object to reduce the power consumption and increase the efficiency of a switching power supply unit or a semiconductor device for a switching power supply.
In order to achieve the object set forth above, the present invention provides a switching power supply unit or a semiconductor device for a switching power supply, in which the application of a switching signal to a switching element is stopped depending on a power supply voltage of a control circuit that is detected by an output voltage detection circuit and fed back to the control circuit.
Specifically, a switching power supply unit of the present invention includes: a smoothing input capacitor for receiving a first DC voltage; a switching element for receiving the first DC voltage; a control circuit for controlling an operation of the switching element; a control circuit power supply capacitor provided between an output terminal of the switching element and a reference voltage terminal of the control circuit; a voltage conversion circuit for receiving an output signal from the switching element and converting the first DC voltage into a second DC voltage whose absolute voltage value is smaller than that of the first DC voltage; an output voltage detection circuit provided on an output side; and a feedback circuit for receiving a detection signal from the output voltage detection circuit and feeding back the received detection signal to the control circuit, the control circuit including: a regulator provided between an input terminal of the switching element and the reference voltage terminal for holding a voltage at the reference voltage terminal at a predetermined value; an output load detection circuit for receiving a feedback signal from the feedback circuit; an error amplifier for producing an error voltage signal that represents a difference between an output signal from the output load detection circuit and a reference voltage; an element current detection circuit for detecting a current flowing through the switching element with an output signal of the error amplifier being a reference; and a light load detection circuit for stopping a switching operation of the switching element when the error voltage signal is lower than a lower limit voltage while resuming the switching operation of the switching element when the error voltage signal is higher than an upper limit voltage, wherein: the light load detection circuit includes a reference voltage source for outputting the lower limit voltage and the upper limit voltage, and a comparator for comparing a voltage signal output from the reference voltage source with the error voltage signal; and the lower limit voltage and the upper limit voltage are switched from one to another based on an output signal of the comparator.
With the switching power supply unit of the present invention, the light load detection circuit receiving the error voltage signal stops the switching operation of the switching element when the received error voltage signal is lower than the lower limit voltage while resuming the switching operation of the switching element when the error voltage signal is higher than the upper limit voltage. When the switching power supply unit is in a light-load operation, the load current decreases and the second DC voltage, which is the output voltage, increases, whereby the feedback signal current increases. This increases the voltage value of the output signal from the output load detection circuit. Therefore, in the error amplifier of the present invention, the difference between the output signal from the output load detection circuit and the reference voltage decreases, and thus the error voltage signal decreases to be lower than the lower limit voltage, thereby stopping the switching operation of the switching element. This reduces the switching loss at the switching element, whereby it is possible to reduce the power consumption during a light-load operation and to improve the power efficiency. Thus, it is possible to significantly reduce the power consumption.
In the switching power supply unit of the present invention, it is preferred that the feedback circuit includes a photo coupler.
In the switching power supply unit of the present invention, it is preferred that the error voltage signal changes linearly according to a current flowing through the feedback circuit, so that a period of time for which the switching operation of the switching element is stopped changes linearly with respect to a change in a power at a main output terminal.
In the switching power supply unit of the present invention, it is preferred that the output voltage detection circuit includes a circuit including a Zener diode and a light emitting element connected in series with each other.
In the switching power supply unit of the present invention, it is preferred that the feedback circuit includes a switching element having a light receiving section.
In the switching power supply unit of the present invention, it is preferred that the output voltage detection circuit and the feedback circuit are a Zener diode and a photo coupler, respectively.
In the switching power supply unit of the present invention, it is preferred that a value of the first DC voltage is about 100 V or more, and a value of the second DC voltage is about 25 V or less.
In the switching power supply unit of the present invention, it is preferred that the control circuit includes overcurrent protection means for detecting an overcurrent to stop the switching operation of the switching element.
In the switching power supply unit of the present invention, it is preferred that the control circuit includes overcurrent protection means for detecting an overcurrent to stop the switching operation of the switching element, and overheat protection means for detecting an overheated state to stop the switching operation of the switching element.
A semiconductor device of the present invention is a semiconductor device for a switching power supply, the switching power supply including: a smoothing input capacitor for receiving a first DC voltage; a switching element for receiving the first DC voltage; a control circuit for controlling an operation of the switching element; a control circuit power supply capacitor provided between an output terminal of the switching element and a reference voltage terminal of the control circuit; a voltage conversion circuit for receiving an output signal from the switching element and converting the first DC voltage into a second DC voltage whose absolute voltage value is smaller than that of the first DC voltage; an output voltage detection circuit provided on an output side; and a feedback circuit for receiving a detection signal from the output voltage detection circuit and feeding back the received detection signal to the control circuit, the semiconductor device including the switching element and the control circuit, the control circuit including: a regulator provided between an input terminal of the switching element and the reference voltage terminal for holding a voltage at the reference voltage terminal at a predetermined value; an output load detection circuit for receiving a feedback signal from the feedback circuit; an error amplifier for producing an error voltage signal that represents a difference between an output signal from the output load detection circuit and a reference voltage; an element current detection circuit for detecting a current flowing through the switching element with an output signal of the error amplifier being a reference; and a light load detection circuit for stopping a switching operation of the switching element when the error voltage signal is lower than a lower limit voltage while resuming the switching operation of the switching element when the error voltage signal is higher than an upper limit voltage, wherein: the light load detection circuit includes a reference voltage source for outputting the lower limit voltage and the upper limit voltage, and a comparator for comparing a voltage signal output from the reference voltage source with the error voltage signal; and the lower limit voltage and the upper limit voltage are switched from one to another based on an output signal of the comparator.
The semiconductor device for a switching power supply of the present invention has a configuration as that of the switching power supply unit of the present invention, whereby effects as those of the switching power supply unit can be obtained. In addition, since the switching element and the control circuit are made into a semiconductor device, it is easy to reduce the size and the power consumption.
It is preferred that the semiconductor device for a switching power supply of the present invention further includes detection voltage variable means capable of variably setting a value of the lower limit voltage or the upper limit voltage.
In the semiconductor device for a switching power supply of the present invention, it is preferred that the switching element and the control circuit are integrated on a single semiconductor substrate so that the input terminal and the output terminal of the switching element, and the reference voltage terminal and a feedback signal input terminal of the control circuit serve as external connection terminals.
In the semiconductor device for a switching power supply of the present invention, it is preferred that the switching element and the control circuit are contained in a single package so that the input terminal and the output terminal of the switching element, and the reference voltage terminal and a feedback signal input terminal of the control circuit serve as external connection terminals.